Acoustic position locating systems are well known in the prior art. Some systems employ a pointer having a spark gap incorporated into its structure. The spark gap generates an acoustic signal which is propagated to orthogonally oriented, linear microphones. The arrival of the spark acoustic signal at a microphone is detected by an amplitude discrimination circuit and then passed to a timing circuit which compares the time of arrival of the signal with the time of the signal's generation, to thereby achieve a range determination Spark-acoustic position determining systems are disclosed in U.S. Pat. Nos. 3,838,212 to Whetstone et al.; 4,012,588 to Davis et al.; 3,821,469 to Whetstone et al.; 4,357,672 to Howells et al.; and 3,731,273 to Hunt. Spark-based acoustic ranging systems exhibit a number of disadvantages. The generated spark creates both possible shock and fire hazard, makes an audible noise which is, at times disconcerting, and creates electromagnetic interference. Further, since the spark generates a shock acoustic wave, its detection is dependent upon the accurate sensing of the leading edge of the wavefront. Due to changes in amplitude as the spark source is moved relative to the microphones, and further, due to the wavefront's non-linear rise time, accurate sensing is difficult to implement reliably. In fact, systems which employ spark gap ranging exhibit a limited distance resolution specifically due to the detection problems which arise from the use of a shock acoustic wave.
A further class of acoustic wave position determining systems employ emitted, periodic, acoustic signals for ranging purposes. For instance, in U.S. Pat. No. 3,504,334 to Turnage, Jr., microphone receivers of the bar type are oriented along X and Y axes, and measure the propagation time of an acoustic signal from a measuring point to the respective receivers. The measured times are converted to minimum distances between the measuring point and the respective receivers, thereby enabling the coordinates of the measuring point to be determined. Bar-type microphones, (used by the Turnage, Jr.) are both expensive and are difficult to apply to limited-size position determining systems (e.g. desk top size). Furthermore, the accuracy of systems which employ bar-type microphones depends on the uniformity of the emitted acoustic wavefront, and if there is any aberration in the wavefront, inaccurate range measurements result.
Another patent which employs bar-type microphones is No. 4,246,439 of Romein. Romein employs a pair of acoustic transmitters mounted on a stylus, which transducers enable the precise position of the stylus tip to be determined.
Others have attempted to overcome the above-stated problems by employing discrete, point microphone receivers. In U.S. Pat. No. 3,924,450 of Uchiyama et al., a three dimensional acoustic range determining system is broadly described and includes three microphones placed about a surface to be digitized. A stylus having two acoustic sources is used to point to various points on the 3-D surface. Signals from the stylus are received by the microphones and analyzed to determine the digital position of the surface point. Little detail is given of the measurement method employed by Uchiyama et al.
Another acoustic point digitizer employing a wireless stylus or puck is described by Herrington et al. in U.S. Pat. No. 4,654,648. In that system, a stylus emits acoustic signals and a linear array of microphones receives the signals and determines the position of the stylus by hyperbolic triangulation. Herrington et al. uses point source acoustic transmitters which enable uniform transmission patterns to be achieved. The measurement technique employed by Herrington et al. that the output from the sensing microphones be selectively switched to feed into a detector circuit, which circuit in addition to including a zero crossing detector, also samples and holds the value of peak amplitudes of each cycle. This data is used to determine range information and from that, to obtain positional triangulation of the acoustic transmitter. While the Herrington et al. system overcomes many prior art problems, its use of a linear array of microphones; the switching of the microphones; and the sampling of the input signals to determine instantaneous amplitudes all present problems which lead to an unnecessarily complex and expensive system.
Accordingly, it is an object of this invention to provide an acoustic position determining system which provides improved position accuracy and detection.
It is another object of this invention to provide an acoustic position digitizing system which is not dependant upon the amplitude of a received acoustic signal.
It is still another object of this invention to provide an acoustic position determining system which enables arbitrary positioning of acoustic receiving units.
It is another object of this invention to provide an acoustic position determining system which is easily calibrated.
It is a further object of this invention to provide an acoustic position digitizing system which employs an open-loop signal processing element for determining the time of arrival of an acoustic signal.